1. Field of the Invention
The present specification relates generally to a method and an apparatus for pattern sequence synchronization between a first and a second pattern sequence. Certain embodiments discussed in the present specification relate to a synchronization between a received pattern sequence which may, for example, be generated at a transmitting device and a reference pattern sequence of a receiving device. Certain embodiments are applicable in single carrier transceivers with frame synchronization for pilot detection, in single carrier quadrature direct conversion transceivers for single carrier detection, and/or in OFDM (Orthogonal Frequency Division Multiplexing) systems for finding training sequences.
2. Description of the Related Art
Frame synchronization is often used to enable further blocks and/or error correction loops in a receiver chain. The reference pattern sequence, usually known at the receiver and typically synchronized by a pilot detection and synchronization scheme, generally gives necessary information to receiver data aided error detector loops, normally for improving the receiver performance.
In continuous transmission based systems, it is often necessary to have an extremely signal distortion tolerant architecture. One of the major errors is the transmitter/receiver carrier-frequency mismatch.
Synchronization of a reference pilot symbol sequence with a pilot symbol sequence in received data is generally done, according to the related art, by correlation of IQ symbols of the received pattern sequence with the reference pattern sequence of pilot symbols. Such a procedure is also typically called a Pilot Vector (PV) correlation procedure.
In the following description of the conventional PV correlation procedure, it is generally assumed that there is no frequency mismatch between the transmitter and the receiver.
The correlation is usually running permanently on the received pilot symbols. For each received pilot symbol s(n), the correlation is commonly done on the last “Z” received symbols (s(n) . . . s(n−Z)), normally with reference pilot symbols (r(1) . . . r(Z)).
                    s        ⁡                  (          n          )                    =                                                  s              I                        ⁡                          (              n              )                                +                                    js              Q                        ⁡                          (              n              )                                      =                                                        s              ⁡                              (                n                )                                                          ·                      ⅇ                                          jφ                s                            ⁡                              (                n                )                                                          ;                  r        ⁡                  (          k          )                    =                                                  r              I                        ⁡                          (              k              )                                +                                    jr              Q                        ⁡                          (              k              )                                      =                                                        r              ⁡                              (                k                )                                                          ·                      ⅇ                                          jφ                r                            ⁡                              (                k                )                                                          ;                      ⁢                            corr          PV                ⁡                  (          n          )                    =                        ∑                      k            =            1                    Z                ⁢                                  ⁢                                            s              ⁡                              (                                  n                  -                  k                                )                                      ·            r                    *                      (                          Z              -              k                        )                                                  ⁢                            corr          PV                ⁡                  (          n          )                    =                        ∑                      k            =            1                    Z                ⁢                                  ⁢                                                        s              ⁡                              (                                  n                  -                  k                                )                                                          ·                                                r              ⁡                              (                                  Z                  -                  k                                )                                                          ·                      ⅇ                          j              ⁡                              (                                                                            φ                      s                                        ⁡                                          (                                              n                        -                        k                                            )                                                        -                                                            φ                      r                                        ⁡                                          (                                              Z                        -                        k                                            )                                                                      )                                                                            ⁢                  Z:            ⁢                          ⁢      Search      ⁢                          ⁢      Sequence      ⁢                          ⁢      Length      
Assuming no imperfections, if the reference pattern sequence matches with the received pattern sequence of pilot symbols and “|s(n)|=|r(k)|=a”, the correlation output corrPV normally results to:
            corr      PV        ⁡          (      n      )        =    ⁢                    ∑                  k          =          1                Z            ⁢              a        ·        a        ·                  ⅇ                      j            ⁡                          (              0              )                                            =          Z      ·              a        2            
The analog stages of a transceiver system typically introduce unwanted imperfections such as, but not limited to, transmitter/receiver carrier-frequency and/or phase mismatch. Normally, the higher the carrier-frequency, the higher the impact to transmitter and/or receiver carrier-frequency mismatch will be. For example, during the system start-up process, an extended mismatch is usually expected. The receiver commonly has to be able to detect and/or correct these effects in the widest range possible. The carrier-frequency mismatch is typically seen as an incremental phase shift (nΔθ) on received IQ symbols s(n).s(n)=|s(n)|·ejφs (n)·ej·n·Δθ
The conventional method according to the related art is usually to correlate the received IQ symbol pattern sequence with a reference pattern sequence of pilot symbols. This approach is normally rather sensitive to carrier-frequency mismatch because each correlation product is generally infected by the mismatch seen as IQ symbol rotation (0 . . . nΔθ). The correlation products commonly calculated over the complete pattern sequence length are normally summed up. Finally, the impact of the carrier frequency mismatch usually distorts the correlation result significantly.
As mentioned above, a carrier mismatch typically has a significant effect to the above-described pattern sequence correlation. In case the carrier mismatch exceeds a certain range the correlation normally does not deliver the necessary periodic maximum peaks. Consequently, the exact position of the pilot sequence and/or the framing will generally not be found. Furthermore, frame-dependent working blocks, as well as detection blocks, which usually work on reference and/or received pilot symbols, will commonly not get the necessary inputs. Even further, the receiver often does not get into the “lock” status, as the pilot sequence is generally not found.
In other words, if the conventional frame synchronization method is used, the digital receiver is commonly unable to start-up above a threshold of mismatch. The related art correlation-based frame synchronization techniques typically tolerate this carrier-frequency offset only up to a certain limit, since the impact of the carrier frequency mismatch normally distorts the correlation result significantly.